(1) Field of the Invention
The present invention relates to a fabrication technique for semiconductor integrated circuits, and more specifically to dry etching of aluminum films, used as part the integrated circuits.
(2) Description of Prior Art
Very large scale integration, (VLSI), has allowed the semiconductor chip industry to reduce cost by increasing chip density, (circuits per chip), while still maintaining reliability. The major factor in increasing chip densities has been the ability of the industry to drastically reduce the image size of specific chip features. Advances in lithography, via use of more advanced cameras, as well as the development of more sensitive photoresist materials, have led the way for microminiturazation. However forming the smaller image pattern in the photoresist still leaves the process engineer with the situation of producing this small image on the semiconductor. One such area where difficulty can arise is the transfer of this small image in photoresist, to an underlying aluminum layer, on the semiconductor.
The use of reactive ion etching, (RIE), has allowed the industry to successfully transfer small images in photoresist, to underlying metal layers. A major advantage of RIE processing is the anisotropic component of this etching technique. The low pressures associated with RIE drastically reduces most isotropic aspects, and therefore allows a less tapered or less undercut profile to be obtained. However the use of RIE also demands that a high selectivity exist between the material being patterned, aluminum in this case, and the underlying material, silicon dioxide in this case. It is also desirable that the removal rate of the masking photoresist be much smaller then the removal rate of aluminum. Therefore the chemistry used with RIE processing has to be carefully selected and developed to successfully produce the images needed with VLSI technologies. One solution, U.S. Pat. No. 5,211,804 by Kobayashi, explains how a specific RIE process achieves metal shapes without much tapering or undercut and also without degradation, or too much removal of the underlying material, silicon dioxide. This is accomplished using a BCl3, Cl2, and 02 chemistry. However in this case the masking or patterning material was not photoresist, but silicon dioxide. (The image was initially formed in silicon dioxide using photoresist as the mask, followed by removal of the photoresist). The use of 02 is critical to this invention for without it the desired characteristics would not have been achieved. However the use of 02 as part of the RIE chemistry demanded the use of an non-organic masking film, silicon dioxide, and all the additional processing and cost associated with the additional steps.
The purpose of this invention is to achieve the desired profile in aluminum, without undercut or tapering, and with a chemistry that exhibits the desired selectivity between aluminum and the masking and underlying materials, using the less expensive process, direct photoresist masking. Therefore a RIE process, without 02 as one of its components, was invented.